Production of furfural



Jan. 2, 1951 A. P. DUNLOP 2,536,732

PRODUCTION OF FURFURAL Filed MaICh 20, 1947 SPE/ll T A /000/9 BY Maf/M @w QZ??? Patented Jan. 2, 1951 PRODUCTION F FURFURAL Andrew P. Dunlop, Riverside, Ill., assigner to The Quaker Oats Company, Chicago, Ill., a corporation of New Jersey Application March 20, 1947, Serial No. 735,934

8 Claims.

. l The present invention relates to improvements vin the manufacture of furfural and more particularly to the method of recovering the furfural from the reaction mixture in which it is produced.

It is well known that various types of organic raw materials containing pentosans are capable of yielding furfural by a reaction which involves the hydrolysis of the pentosans into correspond- Ving pentoses and the dehydration oi the pentoses into furfural.

Y Thus it has been proposed to manufacture furfural from oat hulls, corn cobs, and the like, by either a one or two step process.

In the two step process, the oat hulls, corn cobs and the like are rst treated with water and an acidic catalyst to produce the particular pentose known as xylose, whereafter the solution of xylose is subjected to further heat, preferably under pressure, so as to convert the xylose into furfural.

' It has beeny found that the second step, conversion of pentose into furfural, is complicated by a vsimultaneous reaction in which pentose is transformed into non-furfural yielding products.

In addition, the formed furfural is destroyed, in part, if left in contact with the acidic medium required for its formation.

Kinetic studies have revealed the relative rates of these three reactions at a Variety of conditions of temperature and acidity. F01` example. the relative rates of reactions A and B, as shown below:

A Fuifural P entose B Destruction products Eiect of mylose concentration on furfural yield Temperature, 160 C. Catalyst, 0.1 N HzSO4.

Maximum Obtain- X lose Concentral Stion pam/100 aagc' parts solution theory 0. 39 82 0.78 77 l. 17 74 1. 56 7l 4. 69 57 7. 8l 50 9. 38 49 `Since reactions A and By proceed simultaneously, then in accordance with the tabulated data .ill

it is not possible to attain yields of furfural except at iniinite dilution of the initial xylose. Obviously it is impractical to operate at such dilution, but it is equally obvious that it is desirable to use a fairly dilute xylose solution in order to obtain high yields of furfural. For economical reasons it is preferred to use a solution containing two per cent xylose or less. As will be illustrated later, this can be achieved by use of dilute pentose solutions, or by incremental addithe prior art processes the formed furfural was driven out of the reaction mixture by continuous passage of steam through the reactor. The steam carries off the furfural at a concentration which is determined by the furfural concentration in the reactor; greater steam consumption being required at low furfural concentrations. For the nal separation of pure iurfural it was necessary to resort to rather cumbersome and expensive distillation methods. In the dilute solutions, with which the present invention is concerned, such a ymethod of furfural removal is impractical from a cost standpoint.

It is one of the objects of this invention to carry out the step of conversion of pentose to furfural, whether from pentose itself or from a pentosan-containing material, in a manner so as to avoid at all times a high concentration of pentose in the reactor.

A further object is to carry out the conversion to furfural in the presence of a furfural solvent which has furfural C. at '760 mm.), which solvent also also must have the properties of not forming an azeotropically boiling mixture with furfural, and one which does not appreciablv dissolve in water, and also does not react with furfural.

A further object of the invention is to extract furfural continuously from a reaction mixture in which it is formed by means of a solvent, such as that mentioned in the previous paragraph, followed by the distillation of the furfural from said solvent. and the return of the latter to the reaction mixture.

Further objects of the present invention will become apparent from the further description hereinbelow when considered in connection with the concurrently nled drawing, in which Fl iS a diagrammatic drawing of an appaa boiling point higher than that ofl ratus and process in which the solvent employed has a. specific gravity lower than that of the aqueous medium;

Fig. 2 is a similar drawing, but with the apparatus and process arranged for the use of a solvent which has a specific gravity greater than that of the aqueous medium.

The process may briey be described as consisting of the following steps:

A pentose, or pentosanic material, dissolved or suspended in a sufficient quantity of water containing the necessary amount of acidic catalyst, is continuously introduced into a reaction Vchainber in which suitable conditions of temperature and pressure are maintained to convert either the pentose or the pentosan into furfural, this being preferably accomplished under sucient pressure to maintain both the water, the Vfurfural and the solvent in liquid phase, while at the same time the desired solvent is continuously introduced, which, .coming into counter-current contact with the reaction mixture, will selectively dissolve the furfural therefrom, whereafter 'the mixture of solvent and furfural is removed, preferably continuously, and directed into a suitable still in which the furfural is distilled olf as the overhead product, while the solvent remains as the kettle product, and is recycled to the reaction Zone.

A considerable number of solvents lend themselves to the carrying out of the present invention, provided only that the solvent has the following essential characteristics:

1. It must have a boiling point higher than 160 C. at atmospheric pressure.

2. It must be a good furfural solvent.

3. It must be liquid at a temperature of 100 C. or above.

4. It must not react with the furfural.

5. It must not form an azeotropically boiling mixture therewith.

6. It must be substantially insoluble in water.

7. It must have a specic gravity greater or less than the aqueous medium.

8. It must permit the removal of furfural by steam stripping or permit fractionation of furfural away from the solvent.

Useful in the present connection are for example'the higher boiling aromatics such as diethyl benzene, dipropyl benzene, dimethyl-ethyl benzene, 2-ethyl-2,4dimethyl benzene, butyl benzene, al1 being examples of aromatic hydrocarbons, by which name they will hereinafter be designated. Not suitable are of course aromatic hydrocarbons having a boiling point below `that of furfural (160 0.).

There also be used various halogenated aromatic hydrocarbons such for example as dichlorobenzene, dibromobenzene, bromo-chlorobenzene, and preferably trichlorobenzene, which latter has a boiling peint of about 181 C.

Particularly suitable are certain chlorinated diphenyls, especially those which are sold under the trade designation of Arochlors which latter are described on page 9e of the Condensed Chemical Dictionary, third edi-tion, published by Reinhold Publishing Corporation, New York, 1942. These so-called Arochlors are non-oxidizing materials of from a liquid to a solid consistency, all of them, however, being liquid 100 C. They are usually designated by certain numbers, and the particular kind which has been found very desirable in the present connection is Arochlor No. 1232. The last two numbers of the figure refer to the percentage of chlorine in the cornposition based upon the fact that it is a chlorinated diphenyl substance also known as biphenyl. It is obvious, however, that other types of Arochlors than the No. 1232 may be used, such as those described in the above mentioned Condensed Chemical Dictionary.

All of the materials recited are operative in the present connection as they are non-reactive with furfural, do not form an azeotropically boiling mixture therewith, and are relatively immiscible with, and insoluble in, water.

@ne method of carrying out the present invention may be, for example, as follows:

Thus, 250 parts of a 12% hydrochloric acid solution containing 8 parts of xylose were added to a reaction vessel together with 750 parts of Arocnlor l232; 'the mixture was vheated with agitation and under pressure to a temperature of about C. (plus `or minus 2` C.) and 1.8 parts of xylose and 50 parts of Arochlor 1232 were added to the reaction vessel every twenty minutes. Every four minutes 50 parts of Arochlor 1232 phase were withdrawn from Vthe bottom of the vessel while 50 parts of Arochlor were added at the top. The operation was continued until 41.128 parts of xylose had been added, which theoretically are equivalent to 26.322 parts of furfural. The furfural was recovered from the removed Arochlor material by distillation and was found to constitute an amount equal to 15.408 parts. This was a yield equivalent to about 58.6% of theory. ln addition to the xylose converted into furfural an amount of unchanged xylose equivaient to about 1.2 parts of furfural remained in the residual water in the reaction vessel, as well as about 0.7 part of furfural. This water, of course, could very well be used for making up the next batch so it would not be lost. When this is taken into consideration, the total yield of furfural would have been 64.3% of theory.

It is preferred, however, Vto carry out the operation substantially continuously in a manner which is, wholly diagrammatically, illustrated in the aforementioned drawing. Thus la suspension or solution in water of pentose (while in solution), or of pentosanic material (which would be in suspension), and the proper amount of acidic catalyst such as sulfuric acid, hydrochloric acid, etc., in accordance with well known prior art methods and proportions, is contained in tank 3 from which it is fed by suitable feeding means which are well within the skill of the art (for example pump I1) through the conduit Il into the closed reaction vessel 5, preferably being introduced (see Fig. 1) at a point somewhat below the top thereof, enough being introduced to fill the vessel without leaving too muchY head room therein, enough, however, to take Careof any expansion. At the same time there is introduced from a tank 6 and through the pipe line "i, a sufcient quantity of solvent to constitute from 1 to 15 times the amount of furfuralyielding material that it is expected will be introduced in any given' period. This: solvent is preferably introduced near the bottom of the vessel 5 by means of pump IS andV is allowed to ow upwardly therein, and` therefore counter-currently to the descending mass of reacting material, which latter is withdrawn at the bottom of the reaction Vessel 5 through the valve 8 and pipe 9, The solvent passes upwardly through the reacting mixture within the Vessel 5 and is withdrawn at the top thereof through valve l0 and carried by pipe H to a distilling column I2 in which the furfural will distill overhead passing in the form of vapor through the pipef3 which leads to the condenser I4 in which it is reduced to the liquid condition, eventually accumulating in the furfural-receiver I5.

The residual solvent accumulating in the bottom of the still I2 passes through valve I5 into pipe 'I and is thence pumped back into the bottom of the vessel by means of pump I5. These expedients are of course well known to the art and require no detailed description. In the Aprocess as illustrated in Fig. 1, it is assumed that a solvent is employed which has a specific gravity of less than l, and which therefore will rise upwardly through the reaction mixture in the vessel 5.

Referring now to Fig. 2, an entirely similar process is carried out here except that the reacting material is pumped into the reaction vessel 5 near the bottom thereof instead of near the top and that the solvent is introduced by means of pump I6 from the line 1, near the top of the reaction vessel 5, because in this case the solvent has a speciiic gravity greater than l and formation of pentose into furfural.

will therefore tend to ow downwardly through Y the reaction mixture.

Accordingly therefore the solvent in this case is withdrawn at the bottom of the vessel 5 through pipe II and transferred to the still I2 in which the furfural, in exactly the same manner as has been described in connection with Fig. 1, passes overhead through the vapor line I3 to be condensed in condenser I4 and to accumulate as liquid furfural in the receiver I5.

Re-usable solvent is withdrawn from the bottom of the still I2 and sent through line I and pump I5 into the top of the reaction vessel 5. Spent liquor is withdrawn through valve 3 and line 9, which in this case is at the top of the vessel 5 instead of at the bottom. In either process, however, the flow of the furfural solvent is counter-current to that of the gradually advancing reaction mixture. The spent liquor contains a catalyst and any unreacted pentose as well as pentose destruction products and the like. This can be re-used if desired by suitable means as for example by flashing olf a portion of the spent liquor and adding fresh pentose to make up the desired concentration. If a nonvolatible acid is used as the catalyst l(I-IzSOi, H3PO4, etc.) it would be recovered by this means for re-use.

As a further example of usable conditions it may be stated that an excellent conversion of pentosans to pentoses may be accomplished by heating 6 parts of a pentosan-containing material with 100 parts of 4.4% sulfuric acid solution at 100 C. for fty minutes. By raising the temperature to 121 C. an equally good conversion is obtainable in fty minutes with a sulfuric acid solution of 0.98%; by increasing the acid concentration to 1.9% while maintaining the temperature at 121 C. the time required can be reduced to as little as thirty minutes. The pentose solution thus obtained may be stored in the vessel 3 and .in any other suitable manner, for example by means of the pump I l, introduced into the reaction vessel 5. The conditions which are to be maintained in this vessel are subject to considerable variations, depending upon the rate of reaction desired. For example, fairly complete conversion of pentose or pentosan to furfural may be obtained by heating it to about 160 C. for ve hours when the concentration of the sulfuric acid is about 4.8%. Similar results can be obtained by raising the temperature, and heating at about 210 C. for

fteen minutes, in which case the acid concentration can be reduced to 2.4%. On the other hand if the temperature is lowered to 190 C., the time required for conversion with a 2.2% sulfurie acid must be extended to about sixty minutes.

When employing pentosanic materials as the charge to the reactor, the rst step of the reaction is, of course, the hydrolysis of `pentosans to pentoses. This reaction proceeds, in general, at a more rapid rate than the subsequent trans- Accordingly, in order to avoid the accumulation in the reactor of relatively high concentrations of pentose it is necessary to employ a high liquid-solid ratio at all times; that is a high ratio of aqueous catalyst soluton to oat hulls, corn cobs, cottonseed hulls, or the like. Applicant is aware that it has already been proposed to employ a solvent to remove furfural from a reaction mixture, as is for example described in the Fulmer et al. Patent 2,078,241. In the process as disclosed in this patent, however, the solvent is invariably one which has a lower boiling point than furfural. Toluene is the preferred solvent described in this patent, and this of course has a boiling point of 111 C. while furiural boils at about C. Therefore, when the solution of furfural in toluene is withdrawn in accordance with the teachings of this Fulmer et al. patent, all of the solvent (toluene) must be distilled on, leaving the furfural behind. inasmuch as it is necessary to employ a great deal more solvent .than the amount of furfural to be extracted it is evident that much solvent must be distilled before the furfural is obtained. In the present invention it is only necessary to supply enough heat to distill the furfural leaving the solvent ready for re-use. The heat requirements are therefore greatly reduced when compared with that required by the Fulrner et al. process.

A particular advantage of the present inventionlies in the fact that suicient pressure can he maintained in the reaction Vessel 5 to restrain the volatilization or vaporization of any of the volatile constituents thereof, in other words, the pressure can be high enough to maintain the water, the furfural and the solvents all in the liquid condition.

If for example a temperature is used which is considerably above the boiling point of furfural and that of the solvent, the solution of furfural in the solvent can be withdrawn at a temperature ,l so high that whenV such a mixture is introduced into lower pressure surroundings, as for example in the still or distilling column I2, the furfural will immediately be vaporized, while the solvent, which has a sufficiently high boiling point, will kbe condensed and will flow back to be recycled to the reaction zone. Under these conditions it is often unnecessary to supply separate heat for distilling the furfural.

It will of course be readily understood that the reaction vessel 5 as shown in the two figures of the drawing, is provided with suitable heating means such as steam coils, steam jacket or the like, so that its content may be raised to the desired reacting temperature. Such means are well known and hence are not specifically illustrated. In fact, the method of converting xylans and similar pentosans into pentose andeventually into urfural are so well known that almost any of the hitherto described methods can be employed, adding to such methods the teachings of the present invention, namely the extraction of the furfural from the reaction mixture by means of a solvent which has a higher boiling point than furfural, which does not form an azeotropically boiling mixture therewith, will not react with the furfural, and is substantially immiscible with Water.

The apparatus or" course should be constructed of material which will resist the acidic catalysts and must be strong enough to sustain the desired pressure which may, under certain conditions, run to several hundred pounds per square inch. The construction of such apparatus is however well within the skill of the art, and therefore requires no detailed description.

What is considered new and inventive in the present connection is defined in the hereunto appended claims, it being of course understood that equivalents known to those skilled in the art are to be construed as within the scope and purview of the claims.

Accordingly applicant claims:

l. Process of producing furfural which comprises the step of extracting furfural from a liquid aqueous medium by means of a waterimmiscible furfural solvent having a 1soiling point higher than that of furfural and incapable of forming an azeotropically boiling mixture therewith, and selected from the group consisting of alkylated benzenes, polyhalogenated benzenes, and chlorinated biphenyls.

2. Process of producing furfural which cornprises passing a liquid aqueous solution containing furfural in an extraction Zone counter currently in admixture with a non-water-rniscible furfural solvent having' a boiling point higher than that of furfural and selected from the group consisting of alkylated benzenes, polyhalogenated benzenes, and chlorinated biphenyls, to selectively dissolve the furfural in said solvent, separating the resultingv furfural solution and heating it to a temperature above the boiling point of furfural to distill the furfural therefrom, and condensing the latter.

3. The process as dened in claim 2 in which the urfural solvent after removal of the furfural therefrom is returned to the extraction zone.

4. The process of claim 2 in which the solvent has a specic gravity less than 1.0.

5. The process of claim 2 in Which the solvent has a specific gravity greater than 1.0.

6. Process of producing furfural which comprises passing a mixture of a furfural-yielding material from the group consisting of pentosans and pentoses together with water and an acid catalyst under pressure through a conversion zone maintained at a temperature suitable for the production of liquid furfural from said material, while at the same time passing into and out of said conversion zone a liquid furiural solvent having a higher boiling point than furfural and incapable of forming an azeotropically boiling mixture therewith and selected from the group consisting of alkylated benzenes, polyhalogenated benzenes, and chlorinated biphenyls whereby selectively to dissolve the formed furfural, withdrawing the furfural solution thus formed, and distilling the furfural therefrom.

7. The process as deined in claim 6 in which the solvent is returned to the conversion zone after the furfural has been distilled therefrom.

8. Process of producing furfural which comprises passing a furfural-source material from the group consisting of pentosans and pentoses together with Water and a furfural-producing catalyst into a closed conversion zone in which furfural is produced from said material in the liquid phase, continuously passing through said zone a liquid furfural solvent having a boiling point higher than that of furfural and selected from the group consisting of alkylated benzenes, polyhalogenated benzenes and chlorinated biphenyls, heating the solvent thus passed through the conversion zone to a temperature suicient to distill furfural therefrom, condensing the thus formed furfural vapor to liquid, and returning the residual solvent to said conversion Zone.

ANDREW P. DUNLOP.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,621,441 Suida Mar. 15, 1927 2,078,241 Fulmer et al. Apr. 27, 1937 

1. PROCESS OF PRODUCING FURFURAL WHICH COMPRISES THE STEP OF EXTRACTING FURFURAL FROM A LIQUID AQUEOUS MEDIUM BY MEANS OF A WATERIMMISCIBLE FURFURAL SOLVENT HAVING A BOILING POINT HIGHER THAN THAT OF FURFURAL AND INCAPABLE OF FORMING AN AZEOTROPICALLY BOILING MIXTURE THEREWITH, AND SELECTED FROM THE GROUP CONSISTING OF ALKYLATED BENZENES, POLYHALOGENATED BENZENES, AND CHLORINATED BIPHENYLS. 